1. Technical Field
The present invention relates to a progressive-power spectacle lens design method including a distance portion having power corresponding to distant vision, a near portion having power corresponding to near vision, and a progressive portion provided between the distance portion and the near portion.
2. Related Art
Spectacle lenses are either single-vision spectacle lenses or progressive-power spectacle lenses.
A progressive-power spectacle lens is, for example, an aspheric lens including a distance portion having power (dioptric power) corresponding to distant vision, a near portion having power corresponding to near vision, a progressive portion provided between the distance portion and the near portion, and intermediate side portions provided on both sides of the progressive portion.
The progressive portion provided between the distance portion and the near portion has a progressive surface where addition power continuously changes between a progression start point and a progression end point along a principal meridian that is a collection of points through which the line of sight frequently passes when the lens is worn by a wearer.
Known progressive-power spectacle lenses of related art have a variety of relationships between the length of a progressive corridor, which is defined by the progression start point and the progression end point, and addition power, which is the difference in power between the distance portion and the near portion.
In an example of related art (JP-A-54-85743), a progressive corridor is provided between a progression start point, which is the boundary between a progressive portion and a distance portion, and a progression end point, which is the boundary between the progressive portion and a near portion, and the length of the progressive corridor is set to be longer for greater addition power. In another example of related art (JP-A-64-32227), the distance from the intersection of the boundary between a distance portion and an intermediate portion and a principal meridian to a progression start point on the principal meridian and the length of a progressive corridor are set to be shorter for greater addition power.
In the related art described in JP-A-54-85743, since the lens is so designed that the line of sight passes through the boundary between the progressive portion and the distance portion when the spectacle wearer views an object in front thereof, the spectacle wearer, when moving the line of sight from the distance portion through the progressive portion to the near portion, experiences change in dioptric power at a point on the boundary through which the line of sight passes when the spectacle wearer views an object in front thereof.
The change in dioptric power at a point on the boundary causes the spectacle wearer, if the wearer has been using the progressive-power spectacle lens only for a short period, to feel uncomfortable. As a result, the spectacle wearer will not become readily accustomed to the progressive-power spectacle lens or will feel difficult to use the lens.
In the related art described in JP-A-64-32227, the lens is so designed that the length of the progressive corridor is set to be shorter for greater addition power or changed in inverse proportion to the magnitude of addition power. In the related art described in JP-A-64-32227, when the spectacle wearer is in an advanced stage of presbyopia and requires greater addition power, the change in addition power needs to be greater across the same length of the progressive corridor or the rate of change in addition power needs to be sharply increased. As a result, the spectacle wearer will experience eye fatigue or feel difficult to use the lens.